I fitted a Speedo Healer not long after I bought my 2005 ST1300 last year.
It wasn't overly hard to install but did involve removing the right hand side fairing.
I'm actually not concerned with the possibility of the odometer slightly under-reading now as there was a fair chance it wasn't spot on beforehand as there is, I believe, a +/- 4 % leeway when it comes to required odometer accuracy.
At the end of the day if it does under-read it will have slightly less K's on the clock when it comes time to sell...